The present invention relates to a communication system in which a master station communicates with a group, or a plurality of groups, of remote field stations over one or more full duplex communication links. This apparatus finds particular application in the railroad industry in implementing a centralized traffic control system. Another application of this type of communication system if found in the electric power industry wherein a master station communicates with and controls a plurality of remote field stations.
Taking the railroad application as an example, it was found, many years ago, that increases in efficiency could be obtained by centralizing the control of track switches and track signals in a railroad territory, which may extend for hundreds of miles. Of course, the operator of the centralized traffic system must have continuous up to date information respecting the position of trains in the territory, as well as the condition of track switches and track signals. This is one of the functions traditionally accomplished by the communication system. Additionally, in order to make effective the operator initiated controls, the communication system must transmit the controls from the central office to the appropriate field station in order to actuate the equipment to be controlled.
In the parlance of this field, the data transmitted from the central office, or master station, to an appropriate field station, are termed controls. For purposes of efficient transmission and error checking, the message may be broken up into a number of words, each of which includes an equal number of bits. The first word would normally include the address of a particular field station to which the control is directed. The field stations include receivers which respond to the address of the field station when transmitted to and allow the remaining portion of the message to be received and acted on. The equipment which any field station controls depends, of course, on the configuration of the railroad at that location, including the number of track switches and track signals and the track configuration. The control message must, then be individually configured for each field station and therefor the number of words in a control message may be different for different ones of the field station.
In order to provide the operator at the master station with a picture of the condition of the territory he is to control, information must be transmitted, from each of the field stations, to the master station indicative of the conditions at that location. This data is termed indications and may include the position of railway track switches, the occupancy condition of track sections and the condition of track signals. Generally the indication transmission, that is the transmission from the field station to the master station is initiated by the reception, at the field station, of a message from the master station. This indication message also includes a plurality of words, the first of which may comprise an address, so that the master station is informed of the identity of the field station from which data is being received. The indication message may also include a number of indication words and, of course, the number of indication words depends, in large part, on the railroad apparatus at the field location.
Communication systems for this type of application have a number of requirements in common with many other types of communication systems. One requirement is that indications be transmitted, from each of the field stations, to the master station, at a rate sufficient to give the operator at the master station a "picture" of the railroad condition that is accurate enough for him to act on. In addition, of course, controls must be transmitted at a rate sufficient to ensure that the transmitted control will be received and acted on at the appropriate time. A second requirement is that transmission errors be minimized. Obviously, if an error occurs in an indication message, the operator may be misled as to the condition of the equipment in the field. Similarly, if a transmission error occurs in a control message, apparatus in the field may not be properly operated. Although it is common practice in the railroad industry to equip the field apparatus with safety circuits such that unsafe operations cannot occur, an erroneous control will still require corrective action, needlessly tie up equipment and correspondingly reduce the efficiency of the railroad. In summary, the two requirements of the system are that communications, in both directions, be sufficiently rapid to meet system requirements and, at the same time, be substantially error free.
The prior art illustrates communication systems which have been used in a centralized traffic control system environment which are characterized by extensive use of relays. As compared with present day desires these systems exhibit a need for improvement in two areas. One area is related to speed. The relays are limited in speed of operation by inherent mechanical limitations. The second area in which these systems appear to be in the need of improvement lies in the area of flexibility. It should be apparent, from the above brief recitation of the functions to be performed by the communication system that these systems are complex. This makes the operations of design, manufacture and installation, tedious, time consuming and, therefore expensive. By its very nature the relay system did not allow slight changes in function to be effected by simple changes in equipment. Thus, changes in the field may necessitate the addition or deletion of field stations, changing the number of words in a control or indication message, or the like.
Since each of the field stations is, in effect, a unique communication receiver and transmitter, it may be desirable, and sometimes is necessary, to tailor different communication cycle for different ones of the field stations. By communication cycle is meant the delay period between a first transmission to a particular field station and the second transmission to the same field station. Thus, conditions at one field station may require communications to take place within the period of time t, whereas conditions at a second field station may justify the necessity for communications only within a longer period t.sub.2 (that is t&lt;t.sub.2 or even, t&lt;&lt;.sub.2). Obviously, it may be possible to arrange the communication system to communicate with each station within the shorter period t. Clearly, however, it would be more desirable to arrange the communication system such that the first station's communication cycle is less than or equal to t, while the second station's communication cycle is less than equal to t.sub.z. In this manner unnecessary communications with the second field station are not built into the communication system.
The prior art teaches a number of checking procedures that can be employed by the communication system to verify the reliability of the received data, either indications or controls. Furthermore, it is well known in the art to arrange a communication system such that, if unreliable data is detected, retransmission of that data is requested.
For instance, if a field station determines that transmitted controls are, for some reason, considered unreliable, its indication transmission will contain an indication of the status of the received controls and a request that the controls be retransmitted. During the operation of the communication system, however, a station may be out-of-service for one of a number of reasons. It may be the subject of periodic maintenance, or it may be out-of-service due to an equipment failure. For any of these, or other reasons, the out-of-service station will appear to the master station as if a failure in communications had occurred. The master station may then initiate a number of retries, in an attempt to establish communications with that station. Of course, since the station is out-of-service, the master station will not be successful. However, the time wasted in this attempt can be saved if the master station has means, at its disposal, to detect whether or not a station is out-of-service.
Regardless of the apparatus employed to perform the functions necessary to manage a communication system, it is apparent that modifications to the system can be time consuming due to the complex nature of the system. Furthermore, in some situations it is apparent when a system is installed that the number of active field stations are less than the total number of contemplated field stations. It would be desirable, to arrange the communication system to be capable of operating with the contemplated number of stations. At the same time, however, there is the desire not to bog down the communication system in attempts to communicate with non-existent or non-operative field stations.
Furthermore, communication systems intended to satisfy the applications referred to above usually operate in a sequence wherein the master station addresses a control message to a particular field station which, on receipt of its address will initiate the transmission of indications to the control office. Since the communication link is duplex, controls may be transmitted simultaneous with the transmission of indications. Since it may be necessary to retransmit the control message and furthermore since the system has no way of knowing whether or not a retransmission is necessary until the indications have been received, these communication systems generally monitor the received indications to determine whether or not a retransmission of the control message is desired. However, field station transmission failures may result in the master station hanging up or waiting for an indication transmission that will not occur. This obviously needlessly ties up the communication links and therefore is desirable to provide apparatus to determine that it is no longer useful to monitor the field station link and the control system should move on to other pending communication needs.